If these types of role changes were made optional upon registering in the duty finder, the game-play in the dungeons and raids would become much more dynamic and interesting for the player. This would also provide another layer of player-character uniqueness in the game as a whole. It would also make the duty finder player selection much more broad and at the same time, reducing the time it takes to form a party. I would tank if Dragoons were given the option to register as one. I would also like to see a bard with healing songs. (that would be pretty gnarly)
Of course if these changes were incorporated into the game, new armor's would have to be created to accommodate these new roles.